The invention relates to a brake booster, and more particularly, to a brake booster which assures that an increased output can be obtained reliably in response to a force of depression of a small magnitude which is applied to a brake pedal in the event a braking effort of an increased magnitude is required as in an emergency braking operation.
A brake booster which is arranged to permit an increased output to be obtained even in response to a braking depressing force of a small magnitude as in an emergency braking operation which requires a braking effort of an increased magnitude is known in the art, as exemplified by Japanese Patent No. 3,070,672 disclosing a brake booster in which in response to an input which exceeds a given value, an output gradient is increased subsequently to increase the output, or Japanese Laid-Open Patent Application No. 35,474/1998 disclosing a brake booster in which an operation of a brake pedal is electrically detected, and the output is increased in response thereto.
However, drawbacks are pointed out with conventional brake boosters as mentioned above. Specifically, in the brake booster disclosed in Patent No. 3,070,672, the arrangement is such that the output can be increased only after the input exceeds a given value, and accordingly, the output cannot be increased in immediate response to the quick depression of the brake pedal. On the other hand, in Application No. 35,474/1998, there is a need to provide a solenoid operated valve and a sensor which detects the degree of depression of the brake pedal in an integral manner with the booster, resulting in a complicated arrangement and an increased manufacturing cost of the brake booster.
In view of the foregoing, it is an object of the present invention to provide a brake booster which is simple in construction and which is capable of immediately increasing the output in response to a quick depression of a brake pedal.
Specifically, the present invention relates to a brake booster including a valve body slidably disposed within a shell, a power piston mounted on the valve body, a constant pressure chamber disposed forwardly of the power piston within the shell, a variable pressure chamber disposed rearwardly of the power piston within the shell, and a valve mechanism disposed within the valve body for switchably controlling a supply/discharge of a fluid to and from the variable pressure chamber, the valve mechanism comprising a vacuum valve seat formed on the valve body, a valve plunger connected to an input shaft and disposed within the valve body so as to be axially movable and having an atmosphere valve seat formed on a rear portion thereof, and a valve element including a first seat area which is adapted to be seated on the vacuum valve seat and a second seat area which is adapted to be seated on the atmosphere valve seat. According to the present invention, the brake booster further comprises length adjusting means which reduces the axial size of the entire valve plunger when the input shaft has advanced through a given stroke relative to the valve body.
With this arrangement, it is assured that when the brake pedal is quickly depressed during an emergency braking operation, the input shaft can be advanced through a given stroke or more relative to the valve body from its inoperative position. As a consequence, the length adjusting means reduces the axial size of the entire valve plunger, whereby an atmosphere valve which is formed by the atmosphere valve seat formed on the valve plunger and the second seat area of the valve element is opened to a greater degree than during a normal operation, thus allowing the atmosphere to be rapidly introduced into the variable pressure chamber to allow a rapid rise in the output from the brake booster. Under the balanced condition where both the atmosphere valve and the vacuum valve are closed, a clearance between a reaction disc and the valve plunger increases by an amount corresponding to the reduction in the axial size of the entire valve plunger which is caused by the length adjusting means, and the reaction disc bulges to a greater amount to increase the output in a corresponding manner.
Accordingly, there can be provided a brake booster which is simple in construction and which is capable of immediately increasing the output in response to a quick depression of the brake pedal.
Above and other objects, features and advantages of the invention will become apparent from the following detailed description of several embodiments thereof with reference to the drawings.